Radar has long been employed in applications such as air traffic control, fire control, navigation, etc. Due to the many advantages of radar usage in such applications, radar has also been the subject of continuous improvement efforts. One of the fundamental requirements of many types of radar is the implementation of some form of beam steering in order to conduct a sweep of a particular area in an effort to, for example, detect contacts, targets, navigation aids, etc. Conventional radars typically employed mechanical beam steering methods. For example, a commonly recognized image of a radar antenna is a parabolic antenna mounted on a rotating apparatus which steers the antenna. Such rotating radars often utilize complex mechanical mechanisms such as hydraulics, electric motors or hinge appendages in order to achieve the rotation that provides beam steering. However, mechanical apparatuses such as those listed above often require intensive maintenance in order to ensure optimal performance. Additionally, failure of a single element of rotating radars may render the entire apparatus unusable. Rotating radars also suffered limitations in scanning rates due to the mechanical rotation, which translated into limitations with respect to contact or target detection.
In order to overcome several of the disadvantages of conventional radars, electronic scanning antennas (ESAs) have been developed, which are also known as phased array radars. ESAs are a revolutionary type of radar whose transmitter and/or receiver functions are composed of numerous small modules which may either receive or both transmit and receive. ESA radars perform electronic beam steering which can be done without the limitations caused by physical rotation. Accordingly, ESAs feature short to instantaneous (millisecond) scanning rates. Additionally, since ESAs do not rotate, ESA radars have vastly simpler mechanical designs and require no complex hydraulics for antenna movement or hinge appendages that may be prone to failure. The ESA radar also occupies less space than a typical radar because ESAs have reduced infrastructure requirements as compared to rotating radars. The distributed nature of the transmit function in an ESA also eliminates the most common single-point failure mode seen in conventional rotating radars of lost ability to rotate. Given the improvements above, ESA maintenance crews are far less severely taxed, and the ESA radar is much more reliable than a comparable rotating radar. In addition to having much higher scanning rates than conventional radar, ESAs also typically have a much longer target detection range, higher capabilities in terms of the number of targets that can be tracked and engaged (multiple agile beams), low probability of intercept, ability to function as a radio/jammer, simultaneous air and ground modes, etc.
Although ESA radars represent a significant improvement over conventional radars, there is still a desire to improve the capabilities of ESA radars. In this regard, wideband receive-only microwave phased array antennas, for example, have been introduced into many new applications on naval, airborne and space platforms. However, due to microwave device nonlinearity, ESAs have shown inherent problems with creating spurious signals. For example, spurious signals known as “ghost beams” (i.e., signals that are internally generated and appear to originate at angles and frequencies other than their true origin) are particularly prone to creation in signal rich environments (i.e., environments with relatively large numbers of targets) due to a combination of the signal environment with nonlinear device properties. The ghost beams may result in false alarms, confusion, or other problems since the ghost beams indicate the presence of targets at locations and frequencies where no targets actually exist.
ESAs typically use time delay circuitry in order to demonstrate wider bandwidth and improve directionality of the antennas. Nonlinearities that follow the time delay circuitry cause harmonic responses and intermodulation of detected signals. Accordingly, a resulting superset of perceived signals includes not only signals corresponding to actual signal sources, but also includes harmonics of each signal corresponding to the actual signal sources and additional signals corresponding to the combined effects of each pair of signals corresponding to the actual signal sources (i.e., sum and difference of distinct frequencies). The resultant ghost beams are indiscernible from responses corresponding to actual signal sources and thereby generate clutter that may obscure true sources, scramble signals from the true sources, or masquerade as true sources themselves. In order to deal with ghost beams, conventional systems have attempted to improve microwave device linearity, which requires problematic amounts of direct current (DC) power inserted into the post-time delay circuitry. Additionally, conventional systems have employed very complex channelizing filters.
Accordingly, in light of the discussion above, it may be desirable to provide a method of overcoming the problems related to ghost beam generation in a way which would not require large amounts of DC complex channelizing filters.